Mix chocolate, espresso powder, rum, vanilla, salt, and 1/2 cup water in a large heatproof bowl. Set over a pot of simmering water and whisk until melted. Set aside.
In a separate bowl, using a hand mixer, beat the egg yolks with 1/2 cup of the sugar until you can scrape the bottom of the bowl clean, about 2 minutes. Fold about 1/3 of the chocolate mixture into the yolks with a rubber spatula, then fold the egg mixture into the chocolate remaining in the bowl. Set aside.
Wash the beaters and bowl thoroughly and dry them. Whip the egg whites with the remaining 1/2 cup sugar until they hold soft peaks when the beaters are lifted. Fold 1/3 of the whites into the chocolate mixture with a rubber spatula. Once incorporated, fold the remaining whites.
In a clean bowl, beat the cream until it holds firm peaks. Fold the cream into the chocolate mixture 1/3 at a time using a rubber spatula. Divide the mousse among 8 dessert cups. Chill at least 2 hours or up to overnight.
Put the cookies in a heavy sealable plastic bag. Whack them with a rolling pin to break them up into coarse pieces, then crush them into fine crumbs.
Top each dish of mousse with crumbled chocolate cookies to resemble "soil". Finish with edible flowers. Standing them straight up by inserting the stems into the mousse.